Thanks to the comments, I was able to pinpoint the problem: the database was originally a MySQL/MariaDB. It was converted into an MS SQL database using the tool "ESF Database Migration Toolkit - Pro". During the conversion, the tool used the type "datetime" for DateTime columns, which would work fine for the classic Entity Framework 6. However, EF Core uses the type "datetime2". And that's where the problems came from.

I learned a lot through that.